1. Field of the Invention
The present invention relates to a spiral die assembly suitable for forming tubular body by an extrusion molding.
2. Brief Description of the Related Art
In conventional extrusion molding methods for forming tubular bodies, a spider die which frequently causes spider marks and a straight die which tends to cause uneven thickness in molded products are not employed, but spiral die assemblies as shown in FIGS. 4A and 4B are employed instead. In such a spiral die assembly a spiral groove 3 is formed on the outer surface of a mandrel 2 which is inserted in a main body 1 of the die assembly. Molten resin supplied from an extruder is usually introduced into the mandrel via an inlet 31 and divided its flow path into two to eight flow paths at a dividing inlet 32, and finally moved to a die lip (rightmost end of FIG. 4A or 4B) via the spiral groove 3. Depth of the spiral groove 3 is gradually decreasing toward the die lip. Portion of molten resin is moved between the inner surface of the main body 1 and the mandrel 2 as molten resin being moved in the spiral groove 3 and its flow is adjusted in a reserving portion 33 (see FIG. 4B) and a straight portion (a land portion: a rip gap portion), so that molten resin reaches to the die lip in an adjusted flow. Sometimes, a spiral die assembly having no reserving portions (see FIG. 4A) is used depending on products to be molded.
Except the above-mentioned spiral die assemblies, various dies have been proposed in order to homogenize molten resin at the outlet of the die lip.
1) When a porous pipe is formed out of high viscous material, the material between a cylinder and a mandrel is heated by frictional force in order to disperse strands after passing a breaker plate or in order to adhere and to polymerize powdered material. The frictional heat is obtained by rotating the cylinder. The Japanese laid open patent No. 8-57936 (reference 1) discloses a die arrangement such that a die for extrusion molding and a molded product can be relatively rotated (see FIG. 5). The relative rotation is continuously or intermittently executed and is controlled its rotational mode and degree of the rotation so as to control properties of the molded products.
2) The Japanese laid open patent No. 59-9035 (reference 2) discloses a method to modify a thickness of an extruded pipe by controlling a gap between a die and a rotatable eccentric nipple (mandrel) and also controlling a detected temperature difference in a circumferential direction. In the extruder for forming the pipe, a cross head 62 in which a resin passage between a die 64 and a nipple 65 is eccentrically arranged except at the end of the resin passage (see FIG. 6) and a sensor is arranged to detect temperature of molten resin in the radial direction. The die and the nipple are arranged such that either one of them can be rotated around an axis for preventing uneven thickness in the extruded product.
3) The Japanese laid open patent No. 10-29237 (reference 3) discloses a molding method for forming laminated products out of different materials. Distributed and leaked different materials from spiral grooves can be laminated in a predetermined order by rotating a mandrel equipped with the spirals (see FIG. 7).
Usually in a die assembly, a molten resin flow along the inner surface of the cylinder (hereinafter referred as “land flow”) and molten resin flows in spiral grooves (hereinafter referred as “spiral flow”) are moved to a die lip as a flow ratio between the two molten resin flows being kept in a fixed balance. This fixed balance is very important in order to keep good appearance in the molded pipes. When the balance breaks, defects such as streak, uneven appearance, uneven thickness, uneven strength and the like appear in the molded pipes.
An appropriate balance between the land flow and the spiral flow is empirically determined according to a width of the spiral groove, a depth of the groove, a pitch of the groove and the like. And the appropriate balance is also varied according to a flow rate, a temperature, a visco-elasticity of molten resin and the like. When the balance is varied, weld marks are frequently formed in the molded products, since proper measures against such varied balance are not established. Sometimes molten resin is kneaded in order to eliminate weld marks by raising the temperature and lowering the visco-elasticity of molten resin, but uneven thickness, contamination (die marks etc.) due to thermal degradation of the molten resin and like are caused.
A round die or a flat die is used in the extruder for forming molten and kneaded resin into a desired shape.
A pipe body such as film, pipe, blown container, net and the like are formed through the round die. Performance of the die for distributing molten resin uniformly around a circumference on an imaginary plane, affects quality of the pipe body, except molding asymmetrical product out of molten resin extruded from the die lip. When the molten resin is not distributed uniformly or homogeneously around the circumference, the following defects are caused: (a) uneven thickness, (b) deteriorated appearance (weld mark, streak, unevenness, wrinkle and so forth), (c) dispersion in strength, (d) dispersion in electrical properties, and the like. In addition, the round die has the following problems: difficulty to substitute remaining resin in the groove with different type of new resin, to require a considerable time and a considerable amount of wasted resin for substituting resins.
Since visco-elasticity of molten resin in the die depends on its temperature and pressure, and varies according to its flow rate, a molten resin flow in the die should be controlled variably. But the molten resin flow in a straight portion (land portion) near the die lip should be controlled by a fixed means in order to attain stable and straight flow.
The weld mark (weld line) mentioned in (b) means a fine line appearing in the extruded pipe, in other words “a streaked state” when molten resin being flowed in the grooves are combined but not mixed homogeneously.
These problems cannot be solved in the above-mentioned prior arts (spiral dies shown in FIGS. 4A and 4B and references 1 to 3).